Abstract
This paper considers the problem of downlink (DL) training sequence design with limited coherence time for frequency division duplex (FDD) massive MIMO systems in a general scenario of single-stage precoding and distinct spatial correlations between users. To this end, a computationally feasible solution for designing the DL training sequences is proposed using the principle of linear superposition of sequences constructed from the users' channel covariance matrices. Based on the non-iterative superposition training structure and the P-degrees of freedom (P-DoF) channel model, a novel closed-form solution for the optimum training sequence length that maximizes the DL achievable sum rate is provided for the eigenbeamforming (BF) precoder. Additionally, a simplified analysis that characterizes the sum rate performance of the BF and regularized zero forcing (RZF) precoders in closed-form is developed based on the method of random matrix theory and the P-DoF channel model. The results show that the superposition training sequences achieve almost the same rate performances as state-of-the-art training sequence designs. The analysis of the complexity results demonstrates that more than four orders-of-magnitude reduction in the computational complexity is achieved using the superposition training design, which signifies the feasibility of this approach for practical implementations compared with state-of-the-art iterative algorithms for DL training designs. Importantly, the results indicate that the analytical solution for the optimum training sequence length with the P-DoF channel model can be effectively used with high accuracy to predict the sum rate performance in the more realistic one ring (OR) channel model, and thus, near optimal solutions can be readily obtained without resorting to computationally intensive optimization techniques.
Highlights
Generation cellular systems require to maximize spectral efficiency to satisfy the rapidly increasing demand for wireless data services [1], whilst reducing both the cost and energy consumed [2], [3]
Research on massive MIMO systems focused on the time division duplex (TDD) operation, where the required channel state information (CSI) is obtained by sending a superposition of orthogonal sequences over a length of Tp symbols in the uplink (UL) direction during each coherence interval [5], [6], [8], [13], [14]
ACHIEVABLE SUM RATE ANALYSIS we provide the expressions that accurately approximate the SINRk and downlink sum rate for the massive MIMO system under consideration based on the asymptotic random matrix theory approach in [6]
Summary
Generation cellular systems require to maximize spectral efficiency to satisfy the rapidly increasing demand for wireless data services [1], whilst reducing both the cost and energy consumed [2], [3]. CONTRIBUTIONS AND PAPER FINDINGS This paper addresses the challenge of DL channel estimation in an FDD massive MIMO communication system with single-stage precoding and limited coherence time using a non-iterative approach for the DL training sequence design To this end, the principle of linear superposition, in which the DL training sequences are constructed from the eigenvectors of the K distinct correlation matrices, is proposed, which allows a feasible solution for DL channel estimation to be achieved with a reduced design complexity, thereby avoiding the design of existing training sequences that require computationally demanding iterative algorithms. The proposed design paradigm allows a pragmatic DL training design for an FDD massive MIMO system to be achieved with a significant computational complexity reduction
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